1. Field of the Invention
This invention relates to an optical control element for use in an optical isolator, an optical switch, a light modulator and the like, and more particularly to an optical control element using (Cd, Mn)Te which is a ternary compound consisting of CdTe and MnTe.
2. Description of the Prior Art
In the art of optical data processing, optical communication or the like, various techniques which utilize a material exhibiting magneto-optical effects have been proposed to carry out the time and space control of a light wave transmitted on a waveguide path.
Among the magneto-optical effects, a Faraday effect is known as a phenomenon of causing a transparent magnetic material placed in a magnetic field to exhibit optical activity, wherein the direction of rotary polarization of the material is determined depending upon only the direction of magnetization of the magnetic material irrespective of the direction of propagation thereof. Thus, the reciprocated passage of a light in a waveguide system constituted by a material having a Faraday effect causes its rotation to be doubled.
Such a Faraday effect is advantageously utilized in an optical isolator for, for example, optical communication, an optical disc or the like which uses a semiconductor laser as its light source. The semiconductor laser has a disadvantage of readily exhibiting such a return light phenomenon that a light which has been reflected on the end surface of an optical connector or the like enters the semiconductor again, as compared with a gas laser or the like, to thereby render the oscillation of the laser unstable. In order to accomplish the positive blocking of such return light and ensure the stable laser oscillation, an optical isolator as shown in FIG. 7 has been proposed which uses a Faraday effect element.
More particularly, an optical isolator shown in FIG. 7 is constructed in such a manner that the plane of polarization of a laser beam L1 which has been subjected to linear polarization through a polarizer 1 is rotated by means of a Faraday rotor 2. A rotational angle of the polarization plane is determined depending upon the intensity H of a magnetic field and the length of the Faraday rotor 2. When the length of the rotor 2 is determined to cause the rotational angle to be set at 45 degrees, the polarized laser beam L1 is transmitted through an analyzer 3 rearwardly arranged and having a polarization angle of 45 degrees and delivered to the next step. On the other hand, reflected light L2 is subjected to Faraday rotation of which an angle is 45 degrees by means of the Faraday rotor 3. Thus, it is fully interrupted by the polarizer 1 and prevented from entering a laser source (not shown) in the form of a return light.
An optical isolator for an optical communication system using such a semiconductor laser which has been known in the art typically uses a solid YIG monocrystal material. Accordingly, the optical isolator has a disadvantage of being highly expensive because the YIG monocrystal itself is costly. Also, it has another disadvantage that it is rendered large in size.
An optical isolator may be evaluated with, for example, a performance index 2.theta.F/.alpha., wherein .theta.F is Faraday rotation and .alpha. is a light absorption coefficient. It is known in the art that garnet monocrystal Gd.sub.3-x Bi.sub.x Fe.sub.5 O.sub.12 (x=1.5) replaced with Bi has a performance index as large as 2.degree.-30.degree./dB. Nevertheless, a high performance optical isolator is desired which has a performance index exceeding 45.degree./dB. For this purpose, it is desired to develop a magneto-optical element of a large performance index.
Recently, much attention has been devoted to a ternary compound CdMnTe consisting of CdTe and MnTe, because it is supposed to exhibit both ordinary semiconductive properties and magnetic properties. The compound has a zincblende structure as seen in CdTe. It exhibits properties of II-IV group compound semiconductors in the absence of a magnetic field. When it is placed in a magnetic, Faraday rotation is substantially obtained at a room temperature due to the interaction between 3d localized electrons of M.sup.2 + and charge carriers. However, the CdMnTe compound semiconductor now available is only in the form of a solid polycrystal.